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Biological treatment of high thiosulfate industrial wastewater

page 537

Biological Treatment of High Thiosulfate
Industrial Wastewater
WILLIAM C KREYE, Graduate Student
PAUL H. KING, Professor
CLIFFORD W. RANDALL, Professor
Department of Civil Engineering
Virginia Polytechnic Institute and State University
Blacksburg, Virginia 24061
INTRODUCTION
The treatment of industrial wastewater containing high concentrations of thiosulfate-
related materials presents a challenging problem to the sanitary engineer. A number of
industries rely on reduced sulfur compounds in their manufacturing processes, and
typically their raw wastewaters contain significant concentrations of thiosulfate which must
be oxidized during wastewater treatment. In certain situations these wastewaters may
demonstrate concentrations of thiosulfate which can vary from a few thousand mg/l up to
several hundred gms/1 in a very concentrated wastewater stream.
Since thiosulfate has a very high biochemical oxygen demand, thiosulfate must be
oxidized or reduced by either a chemical or a biological process to remove the BOD from
the wastewater. Biological oxidation of thiosulfate may be accomplished, but the
concurrent sulfuric acid production has a strong tendency to lower the pH of the biological
system. This low pH is in fact necessary for the most effective biological oxidation of
thiosulfate to take place. However, the low pH has an adverse effect on the hetrotrophic
microbiological population found in activated sludge. Under low pH conditions, the
autotrophs associated with thiosulfate oxidation will predominate while the hetrotrophic
bacteria responsible for organic BOD removal will die off. The net result will frequently be a
biological unit with an excellent capacity to oxidize thiosulfate but with a greatly reduced
ability to remove organic BOD.
The purpose of the study reported in this paper was to develop a biological system
capable of achieving a high degree of thiosulfate removal from a typical industrial
wastewater. In addition removal of organic matter was another goal of the total treatment
process. Although the concentration of organics in the wastewater selected for this study
was low, organic removal in the presence of a high concentration of thiosulfate was
monitored since it seemed desirable to develop a biological system that would be capable of
treating a mixed industrial thiosulfate waste and domestic sewage in one unit.
PREVIOUS STUDIES
The oxidation of thiosulfate by bacteria of the Thiobacilli species is reported in the
microbiology literature in many places (1,2,3). The Thiobacilli are a relatively small group
of microorganisms who are capable of deriving their total energy requirements from the
oxidation of reduced sulfur compounds. They use carbon dioxide as the source of carbon in
the synthesis of cellular material. The Thiobacilli with the exception of T. novellus which is
facultatively autotrophic are obligate chemoautotrophs. The different strains vary in their
tolerance toward acid conditions, but one species, T thiooxidans, is tolerant of an
extremely acid environment. It is reported to be capable of growing at negative pH values
(4).
With some strains of Thiobacilli, high concentrations of thiosulfate (1 percent) exert an
inhibitory effect. Other strains can tolerate very high concentrations, but the time required
to complete the oxidation of thiosulfate is greatly increased (4). When Thiobacilli are grown
537

Biological Treatment of High Thiosulfate
Industrial Wastewater
WILLIAM C KREYE, Graduate Student
PAUL H. KING, Professor
CLIFFORD W. RANDALL, Professor
Department of Civil Engineering
Virginia Polytechnic Institute and State University
Blacksburg, Virginia 24061
INTRODUCTION
The treatment of industrial wastewater containing high concentrations of thiosulfate-
related materials presents a challenging problem to the sanitary engineer. A number of
industries rely on reduced sulfur compounds in their manufacturing processes, and
typically their raw wastewaters contain significant concentrations of thiosulfate which must
be oxidized during wastewater treatment. In certain situations these wastewaters may
demonstrate concentrations of thiosulfate which can vary from a few thousand mg/l up to
several hundred gms/1 in a very concentrated wastewater stream.
Since thiosulfate has a very high biochemical oxygen demand, thiosulfate must be
oxidized or reduced by either a chemical or a biological process to remove the BOD from
the wastewater. Biological oxidation of thiosulfate may be accomplished, but the
concurrent sulfuric acid production has a strong tendency to lower the pH of the biological
system. This low pH is in fact necessary for the most effective biological oxidation of
thiosulfate to take place. However, the low pH has an adverse effect on the hetrotrophic
microbiological population found in activated sludge. Under low pH conditions, the
autotrophs associated with thiosulfate oxidation will predominate while the hetrotrophic
bacteria responsible for organic BOD removal will die off. The net result will frequently be a
biological unit with an excellent capacity to oxidize thiosulfate but with a greatly reduced
ability to remove organic BOD.
The purpose of the study reported in this paper was to develop a biological system
capable of achieving a high degree of thiosulfate removal from a typical industrial
wastewater. In addition removal of organic matter was another goal of the total treatment
process. Although the concentration of organics in the wastewater selected for this study
was low, organic removal in the presence of a high concentration of thiosulfate was
monitored since it seemed desirable to develop a biological system that would be capable of
treating a mixed industrial thiosulfate waste and domestic sewage in one unit.
PREVIOUS STUDIES
The oxidation of thiosulfate by bacteria of the Thiobacilli species is reported in the
microbiology literature in many places (1,2,3). The Thiobacilli are a relatively small group
of microorganisms who are capable of deriving their total energy requirements from the
oxidation of reduced sulfur compounds. They use carbon dioxide as the source of carbon in
the synthesis of cellular material. The Thiobacilli with the exception of T. novellus which is
facultatively autotrophic are obligate chemoautotrophs. The different strains vary in their
tolerance toward acid conditions, but one species, T thiooxidans, is tolerant of an
extremely acid environment. It is reported to be capable of growing at negative pH values
(4).
With some strains of Thiobacilli, high concentrations of thiosulfate (1 percent) exert an
inhibitory effect. Other strains can tolerate very high concentrations, but the time required
to complete the oxidation of thiosulfate is greatly increased (4). When Thiobacilli are grown
537